where in src/ to put an arch-specific kernel+libc .c file?

Hi all,
Suppose I have an arch-specific .c file and I want its symbols available
in both libc and the kernel. Where in /usr/src would I put such a file?
And how would I connect it to both the libc and kernel builds?
/usr/src/common/lib/libc/arch seems like the right place, but I only see
assembly source code in there, and it's not immediately obvious how the
build system is made aware of those files.
Thanks,
Richard